One conventional means of starting a modern turbofan engine is via a compressed air starter where no dedicated cooling of such an air starter is required, as the heat rejection from such an air starter is low.
Another conventional method of starting more electric gas turbine engines is via an electric driven starter. Here the starter is a variable frequency starter/generator that has a built-in pump for pumping oil through the starter to a dedicated closed circulating oil cooling circuit to transfer the heat generated from the starter to the oil in the oil circuit. The heated oil is then cooled by a flow of fuel via a heat exchanger. The fuel is pumped to the heat exchanger and returned to a fuel tank housed within the aircraft (conventionally known as a fuel return to tank cooling system). This method relies on the fuel mass in the aircraft's fuel tank acting as an external thermal sink to remove heat from the VFSG oil circuit. Because there is a finite amount of fuel, this system is limited to the amount of heat that it can dissipate. In addition, there are problems in the fuel balance within aircraft fuel tanks and fuel tank contamination.
In principle, the thermal mass of the fluids and metals in the cooling circuits could be used as a simple static thermal sink, however, this method is of limited use due to the limited thermal capacity in the cooling circuits. Increasing the fuel volume in the loop would increase the thermal sink capacity, which means a large amount of fuel being stored on engine, and hence, adversely increases the weight of the engine and causes an unnecessary fire hazard for the engine.
Therefore it is an object of the present invention to provide an improved cooling system for the starter/generator that is not hazardous and is acceptable for engine certification requirements.